1. Mince garlic in a food processor. Add avocado, mayonnaise, lemon juice, 2 tablespoons milk, and wasabi powder; purée until creamy, adding remaining milk if needed. Season with salt and pepper. Transfer to a glass bowl, cover, and refrigerate. (Makes about ¾ cup.)
2. In a medium skillet, heat sesame oil until very hot. Place salmon in pan, skin-side up, and cook until lightly browned, about 1½ minutes. Turn fillets over and sprinkle with sesame seeds. Add tamari, toasted sesame oil, and water to pan. Cover and cook over medium heat for 7-10 minutes, until just cooked through, adding more water if needed. Remove from pan, arrange on serving plates, and drizzle with sauce. Serve immediately.